Some flexible printed circuit boards have a conductor circuit pattern printed on a flexible insulating substrate, and a plastic film having a thickness of about 25 to 125 .mu.m is used as such an insulating substrate. Depending on the working conditions, a reinforcement is adhered to the insulating substrate to provide a mechanical reinforcement.
However, if the CTE (Coefficient of Thermal Expansion) of the reinforcement material differs from that of the insulating substrate for a printed circuit board, the difference of the CTE causes mechanical stress between two layers which warps the printed circuit board, resulting in troubles in the assembly of electronic components such as ICs. Accordingly, the reinforcement material should be selected from the materials which have the same or close CTE as that of the insulating substrate.
However, the inventors' experience shows that even a flexible printed circuit board reinforced with a reinforcement having the same material as that of the insulating substrate used causes warp when heated. If the printed circuit board is large sized, the warpage is also so large that the electronic components can hardly be assembled.
It can be presumed that the warp occurs by a bending moment M produced when a residual stress occurring in the interface of the printed circuit board with the reinforcement at a position apart from the center of the thickness of the laminate (printed circuit board to which the reinforcement has been adhered) is relaxed upon heating (M=f.multidot.t wherein t is a distance between the position at which the residual stress occurs and the center of the thickness of the laminate and f is a residual stress).